Heterogeneous multistep synthesis

Another example of the substitution of classic routes for chemical synthesis by multistep catalytic processes is the Rhodia vanillin process (Figure 9.10), which involves four steps, all employing a heterogeneous catalyst. 

Several heterogeneous catalysts have been shown to effect related multicomponent couplings. These include cross-linked polymeric ionic liquid material-supported copper (Cu-CPSIL), silica-dispersed CuO (CuO/Si02), and imidazolium-loaded Merrifield resin-supported copper (Cu-PSIL), all of which can be used in water at room temperature to arrive at 1,4-disubstituted-1,2,3-triazoles from alkyl halides, NaNs, and terminal alkynes. Each can be filtered and reused several times with minimal loss of efficacy. Multistep flow synthesis, specifically including generation of underused vinyl azides and their subsequent click conversions to vinyl triazoles, has also been reported. ... 

The enantioselective synthesis of (R)-roHpram 23 was realized by Tsubogo et al. (2015N329) in a multistep chemical transformation under flow conditions without isolation of any intermediates, using only heterogeneous catalysts (Scheme 5, Figure 2). 

Today, the synthesis of CLA may foUow various ways it can be achieved by alkaline isomerization, by dehydration of methyl ricinolate, from methyl santalbat or soybean oil, by chemoenzymatic or heterogenous catalysis, by the action of microorganisms or through multistep conventional reaction sequences. A short summary will be given here. 

Most of the catalysts have been employed for the synthesis of known hydroxyl- or alkoxycoumarins. Application of heterogeneous catalysts has resulted in environmentally benign, high-yielding experimental techniques, with considerable reduction in acidic and toxic wastes. The potential of these catalysts can be applied to other acid/ base-catalyzed reactions and construction of structurally analogous heterocyclic systems. The ability of these catalysts needs to be exploited for the synthesis of commercially available coumarins with the aim of reducing manufacturing costs. Lastly, their application to the synthesis of unknown coumarins will prove their utility in multistep transformations. 

From an economical and environmental point of view, one pot synthesis of amines from alkenes is a superior pathway over conventional multistep pathways. Hydroaminomethylation is an attractive tool for the synthesis of amines and has been used in the pharmaceutical and chanical industries. Blum developed a heterogeneous system based on [Rh(cod)Cl]2 immobilized within a sol-gel matrix as a highly efficient, regioselective, and recyclable catalyst for hydroaminomethylation reaction of vinylarenes with aniline derivatives (anilines or nitrobenzenes) under mild conditions. 

The one-pot multistep reaction for indole synthesis has received considerable attention since the development of palladium-catalyzed methodologies [17]. Basically, there are three general routes towards the one-pot synthesis of indole catalyzed by heterogeneous supported transition metals (Scheme 5). 

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